Cyclic Threshold Shear Strains in Soils

Based on a synthesis of published laboratory data, two types of cyclic threshold shear strain are examined and their approximate magnitudes identified for different types of soils. They are the linear cyclic threshold shear strain, ${\gamma }_{\mathrm{tl}}$ and the volumetric cyclic threshold shear strain, ${\gamma }_{\mathrm{tv}}$, with ${\gamma }_{\mathrm{tv}}>{\gamma }_{\mathrm{tl}}$. These strains represent boundaries between fundamentally different categories of cyclic soil behavior. For cyclic strains below ${\gamma }_{\mathrm{tl}}$, soil behaves essentially as a linearly elastic material. Between ${\gamma }_{\mathrm{tl}}$ and ${\gamma }_{\mathrm{tv}}$, soil becomes markedly nonlinear but remains largely elastic because permanent changes of its microstructure still do not occur or are negligible. Above ${\gamma }_{\mathrm{tv}}$, soil becomes increasingly nonlinear and inelastic, with significant permanent microstructural changes taking place under cyclic loading. That is, ${\gamma }_{\mathrm{tv}}$, = the threshold separating cyclic strains that cause or do not cause significant permanent changes of soil microstructure. In practical terms, these microstructural changes are manifested in residual cyclic pore‐water pressures in fully saturated soils and permanent volume change in dry or partially saturated soils. Both ${\gamma }_{\mathrm{tl}}$ and ${\gamma }_{\mathrm{tv}}$ depend on the type of soil and are correlated to soils plasticity index (PI). Based on these correlations with PI, a new rational categorization of cyclic shear strains with respect to their magnitude and type of soil is proposed. This categorization enables selection of appropriate type of soil dynamics analysis for given soil and anticipated level of cyclic shear strain.